All three cases they reported involved patients trying to get into trials of novel chemotherapy agents.

The first patient had an albumin level of 22 g/L but needed to be at 30 g/L to get into the trial and thus took a transfusion of 25 percent albumin.

The second patient fell just under the required platelet count of 50,000 per cubic millimeter with a pre-transfusion count of 48,000.

The physician of a third patient ordered transfusion of a unit of red cells to raise her hemoglobin level from 8.3 g/dL to the required 9.0 g/dL for trial enrollment.

These cases might have been handled otherwise, Callum's group noted.

Clinicians should try to correct the underlying laboratory value through other treatments first, such as treating anemia, they urged. Explore all possible other options, including looking for other trials at other institutions for patients who don't meet eligibility criteria, they added.

"Patient safety must trump all decisions for such patients," the group concluded. "There should be few situations, if any, in which a patient receives a transfusion solely for the purpose of temporarily altering a laboratory value to gain admittance to a clinical trial."

Callum and colleagues also offered suggestions for clinical trial design to help prevent such ethical dilemmas.

Researchers and institutional review boards need to carefully consider eligibility criteria in trial design, being alert to laboratory values that could be manipulated by transfusions, they recommended.

"If criteria are listed, there must be a medical reason for excluding patients on the basis of these values," they wrote in the letter, "if clinicians believe that a specific criterion is arbitrary, with no medical justification, they may be tempted to provide transfusions that will help patients meet that criterion."

Trials also need to make clear in the eligibility criteria whether it may be met with the use of transfusions, and publication of the trial results needs to make clear to readers what proportion of patients received transfusion to qualify for enrollment, they added.